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Learn Cosmology

Follow the evidence for how the universe began, grew and what it is made of.

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Cosmology studies the universe as a single system: its origin, its expansion, its contents and its likely future. The field became a precise science in the twentieth century once astronomers found that distant galaxies recede faster the farther away they are, and later detected the faint microwave afterglow of the hot early universe. Beginners start with redshift and the Hubble-Lemaitre law, then move to the Big Bang model, dark matter and dark energy. Learnr shapes a cosmology course around your goals and teaches it through brief, interactive lessons that adapt to you.

How to learn Cosmology, step by step

Get a feel for cosmic distances. Climb the distance ladder: parallax for nearby stars, Cepheid variables for nearby galaxies, and Type Ia supernovae for distant ones. Knowing how each rung calibrates the next explains why cosmic distances carry real uncertainty.
Understand expansion, not explosion. Learn that space itself stretches, carrying galaxies apart, and that redshift grows with distance. Use the scale factor and the raisin-bread analogy to see why every observer sees the same pattern and why there is no center.
Study the evidence for the hot Big Bang. Learn how Penzias and Wilson found the cosmic microwave background in 1965 and why its near-perfect blackbody spectrum matters. Then see how predicted amounts of hydrogen, helium and lithium from nucleosynthesis match what we observe.
Weigh the case for dark matter. Plot a galaxy rotation curve and compare it with what visible matter alone predicts. Add galaxy cluster dynamics, gravitational lensing and the Bullet Cluster, and you will see why most cosmologists accept unseen mass rather than a tweak to gravity.
Learn the Friedmann picture and dark energy. Meet the Friedmann equations conceptually: matter, radiation and a cosmological constant each change how the scale factor grows. The 1998 supernova results showed expansion speeding up, which is where dark energy enters.
Keep the timeline fresh with short reviews. Cosmology has many epochs, from inflation to recombination to the first stars. Reviewing that timeline a few minutes a day in Learnr helps the order and the physics behind each era stay put.

What you'll learn

Explain cosmological redshift and use the Hubble-Lemaitre law to relate distance and recession speed
Describe the evidence for the Big Bang, including the cosmic microwave background and light-element abundances
Explain why galaxy rotation curves point to dark matter
Describe how Type Ia supernovae revealed an accelerating expansion and dark energy
Outline the Lambda-CDM model and the main open questions it leaves

What's inside your Cosmology course

A sample path Learnr builds for Cosmology. Your real course adapts to your goal and level.

1. The scale of the universe and the cosmological principle
2. Redshift, Hubble's law and the expanding universe
3. The hot Big Bang and the cosmic microwave background
4. Big Bang nucleosynthesis and the first elements
5. Dark matter and dark energy
6. Inflation, the Lambda-CDM model and open problems

How long does it take to learn Cosmology?

A solid conceptual grasp of modern cosmology takes about 1 to 3 months at 3 to 4 hours a week. A quantitative treatment with the Friedmann equations is usually an upper-level undergraduate course and assumes calculus plus introductory physics, so it takes longer.

What you need before you start

Curiosity and basic algebra are enough for the concepts. For the mathematical version, you need calculus, Newtonian gravity, and some familiarity with special relativity and thermal physics.

Common mistakes to avoid

Imagining the Big Bang as an explosion at one point in space. It was a rapid expansion of space everywhere at once, with no center or edge.
Thinking redshift from cosmic expansion is just the ordinary Doppler effect. It comes from space stretching while light travels, which matters at large distances.
Lumping dark matter and dark energy together. One adds gravitational pull that holds galaxies together, the other drives accelerating expansion.

How Learnr builds your course

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Cosmology questions, answered

How old is the universe?

Measurements of the cosmic microwave background, interpreted with the standard Lambda-CDM model, give an age of about 13.8 billion years. That figure depends on the model, which is why cosmologists keep testing it with independent data.

Is cosmology the same as astronomy?

Cosmology is a branch of astronomy and astrophysics that treats the universe as a whole. Astronomy also covers individual planets, stars and galaxies in their own right.

What is the Hubble tension?

It is a disagreement between the expansion rate measured from nearby supernovae and the rate inferred from the cosmic microwave background. Researchers are still working out whether it reflects measurement errors or new physics.